Researchers at the University of Alabama at Birmingham say smoking synthetic marijuana — known as Spice or K2 — has been linked to kidney injury.

UAB researchers conducted four case studies involving four young men who smoked synthetic marijuana and suffered kidney injuries afterward. Researchers say the men visited the hospital weeks after using the drug and suffered from abdominal pain, nausea and vomiting.

This week, President Obama signed a law banning synthetic marijuana and other synthetic drugs. Dozens of states and local governments have already tried to outlaw fake marijuana, which has been blamed for hundreds of emergency room visits and a handful of fatalities.

But the bans have proved largely ineffective, and there are fears that the federal law won't be any different.

Synthetic marijuana looks a bit like dried grass clippings. It's readily available on the Internet and in convenience stores and smoke shops, where it's sold as herbal incense or potpourri.